Proposals have been made to construct a solar energy power station by providing a collector in the form of a solar pond, the lower layer of which fuctions as a heat storage zone. A solar pond is a body of water whose upper part is constituted by a halocline which is a layer of water 1 to 2 meters deep having a salinity profile that increases monotonically with depth. When the profile is sufficiently steep, a temperature rise in an underlying stratum of the halocline due to absorption of solar radiation does not decrease the density of this layer relative to the density of the overlying stratum in the halocline sufficiently to effect an exchange of brine between the two layers. Thus, convection in the halocline due to temperature differences is suppressed; and the halocline absorbs solar radiation developing a temperature profile that matches the salinity profile.
Below the halocline is a layer whose salinity is uniform and equal to the salinity at the bottom of the halocline. In many cases, this layer is six meters or more in depth and heat from the halocline is absorbed by this layer due to conduction across the interface. The depth of the heated layer, termed the heat storage layer, depends on the rate of heat input from the halocline due to solar absorption, the rate of heat loss to the ground containing the solar pond, and the rate of heat extracted from the heat storage layer. The temperature of the heat storage layer often will reach 80-90 degrees Celcius and will be uniform throughout the layer by reason of convection currents established by the extraction of heat from this layer. The halocline above the heat storage layer serves, not only as an absorber of solar radiation which inputs heat to the heat storage layer, but as an insulator that effectively insulates the heat storage layer from conductive heat loss to the ambient environment above the halocline.
By reason of environmental conditions of wind and perhaps precipitation, the halocline is covered by a layer of water of uniform salinity equal to the salinity at the top of the halocline. This upper layer is conventionally termed the wind-mixed layer of the solar pond and is convective. In an actual solar pond, the temperature of the wind-mixed layer will generally be a few degrees Celcius above ambient temperature, heat absorbed in this layer serving to establish convection currents that dissipate such heat to the ambient environment. As is well known, the halocline can be stabilized against diffusion of salt and wind mixing by expedients which produce a falling, a rising, or a standing pond.
In a power station in which the heat storage layer of the solar pond is the heat source, proposals have been made to use the wind-mixed layer, as the heat sink. The thermal head involved in such a construction may range from 40-60 degrees Celcius and a suitable power plant for utilizing such a relatively small thermal head is a closed, Rankine cycle power plant utilizing an organic fluid such as Freon as a working fluid. Such a power plant is disclosed in U.S. Pat. No. 3,393,515, and comprises a boiler in the form of a heat exchanger through which brine from the heat storage layer is pumped for vaporizing the working fluid, a prime mover such as a turbine to which the vaporized working fluid is applied, and a condenser, also in the form of a heat exchanger, into one side of which the turbine exhausts. The condenser is cooled by water from the wind-mixed layer, and condensate in the condenser is returned by a pump to the boiler. A power station, operating from a low grade heat source, is hereinafter referred to as a power station of the type described.
The work generated by the heat engine of the prime mover (in this example, a turbine) per unit mass flow of brine from the heat storage layer passed through the boiler will depend on the thermal head of the system, i.e., on the difference between the temperature of the hot brine entering the boiler, and the temperature of the wind-mixed layer. By reason of the absorption of solar radiation by the wind-mixed layer during daylight hours, and heat conducted into the wind-mixed layer, taken together with environmental conditions of wind and ambient temperature, as well as variations in the load imposed on the turbine, the temperature of the wind-mixed layer will have a diurnal fluctuation. During the early afternoon, when the sun is hottest, the wind-mixed layer will have a temperature some 4 to 5 degrees Celcius higher than the twelve hours earlier when the night is coolest and radiation cooling of the wind-mixed layer is maximized. On the other hand, the temperature variation of the heat storage layer in the same two periods of time will vary by a much smaller amount, say one degree Celcius with the result that the power plant is less efficient during hot, daylight hours of operation than during cool, night hours. Obviously, this is not a desirable situation.
It is therefore an object of the present invention to provide a new and improved solar energy power station of the type described wherein diurnal variations in efficiency are significantly reduced.